Nuclear fusion

Summary: The nuclear fusion process is a reaction in which multiple atomic nuclei are combined. This process can produce an enormous amount of energy, which has attracted much attention for its potential applications. The first successful human-initiated demonstration of nuclear fusion came with the development of the powerful hydrogen bomb. Scientists have also explored controlled nuclear fusion as a method to efficiently generate electricity, which would not produce greenhouse gas emissions like fossil fuels and would produce far less radioactive byproducts than nuclear fission reactors.

The process of building up larger atomic nuclei by combining smaller nuclei is called nuclear fusion. This process produces the greatest energy with the lightest nuclides, such as hydrogen and helium. In nuclear fusion, as in nuclear fission, the total mass of the resultant products is less than the original mass. This loss of mass appears in the form of released energy.

In the 1930s, nuclear physicist Hans Bethe first recognized that stars produce energy by fusing hydrogen nuclei to form deuterium. This work led him to receive the 1967 Nobel Prize in Physics. After World War II, having learned of the power of the atom and building the atom bomb, which used nuclear fission, scientists tried to design a nuclear fusion bomb, the so-called hydrogen bomb. They also tried to control nuclear fusion in order to use it to produce electricity.

First Hydrogen Bomb

The first hydrogen bomb, code-named Ivy Mike, was detonated by the United States at Eniwetok Atoll, part of the Marshall Islands, on October 31, 1952. It had a yield comparable to 10.4 megatons of TNT. The mastermind behind this device, Edward Teller, observed the event on a seismograph in Berkeley, California. He observed a tremor in the earth that was produced by shock waves generated thousands of miles away in the Marshall Islands by the explosion. He could not resist the temptation to inform his colleagues in Los Alamos about the success and sent a coded telegram message, "It’s a boy."

Deuterium (D) and tritium (T), both isotopes of hydrogen, are the two most popular raw ingredients for nuclear fusion. A deuterium nucleus contains one proton and one neutron. It is commonly found in nature in the form of heavy water. Out of 6,500 hydrogen atoms, one is in the form of deuterium. In ocean water, every liter has about 33 milligrams of deuterium. One cubic kilometer of ocean water has enough deuterium to generate energy equivalent to about 1.36 trillion barrels of oil, about the total global oil reserves.

Tritium is available in trace amounts in nature. It is usually produced by reacting neutrons with lithium. In a tokamak assembly using D-T reaction, neutrons are produced that are completely unaffected by the presence of a magnetic field. They move unobstructed and hit the surrounding wall that contains lithium and produce tritium. The following are some typical nuclear fusion reactions (MeV stands for mega-electron volts):

¹H + ²H g ³He + 5.49 MeV

²H + ²H g p + ³H + 3.3 MeV

²H + ³H g ⁴He + n + 17.6 MeV

When a atom and a tritium atom are fused together, helium is formed, with the of a neutron and 17.6 million electronvolts (MeV) of energy. To undergo fusion, the interacting nuclei need kinetic energies on the order of 0.7 MeV to overcome the electrical repulsion between their positive charges. These energies are available at temperatures around 10 million kelvins (18 million degrees Fahrenheit), which poses a major problem for containment of the fusion fuel. In some experiments, the resulting plasma generated by heat from an electrical is contained in the reactor core with appropriately shaped magnetic fields (magnetic confinement). In other experiments, fusion is initiated through the heating and compressing of pellets of the light nuclei with a high-intensity laser beam (inertial confinement).

A fusion reaction is about 4 million times more energetic than a chemical reaction using fossil fuels such as coal, oil, or natural gas. While a 1,000-megawatt coal-fired power plant requires 2.7 million tons of coal per year, a fusion plant will only require 250 kilograms of fuel per year, half of it deuterium and half of it tritium. Although each fusion releases less energy than a fission reaction, the energy per unit mass (or per nucleon) is 3.5 times greater for D-T fusion than in uranium fission.

Fusion Power

Although a commercial reactor for electricity production was constructed in less than a decade after the discovery of nuclear fission, a successful commercial reactor using nuclear fusion proved to be a much more elusive goal. Scientists struggle to bring nuclei close enough for fusion in a controlled manner. Accelerators are not good for achieving fusion, since the scattering of nuclei due to Coulomb force is more probable than fusion. Atoms are dissociated into nuclei and electrons at high temperature. This hot ionized gas, with free electrons and positively charge nuclei, is known as plasma. For the fusion of nuclei, a temperature about 10⁸ kelvin (about 10 billion kelvin) must be maintained for a long enough period (for seconds) in a small, confined space for this plasma to have high density. By comparison, the sun’s surface temperature is about 15.7 million kelvin. Because of the high temperature necessary for fusion, fusion devices are often called thermonuclear devices.

Such temperatures and ion density occur in the interiors of stars where nuclear fusion reactions are common. The sun is an example: The gravitational field of the sun creates a confined space with high temperature and enables the sun to fuse 600 million tons of hydrogen into helium every second, releasing an enormous amount of energy. However, without the benefit of gravitational forces in laboratory settings, achieving fusion is a major challenge.

The plasma loses substantial energy at such a high temperature as a result of radiation (bremsstrahlung) and heat conduction to the device boundaries. It is therefore necessary to compensate for this energy loss on an ongoing basis. For sustained fusion to occur, the plasma temperature should be above 10⁸ kelvin and its density should be about 10²⁰ particles per cubic meter (about one-millionth of the density of air). This relationship was defined by J. D. Lawson in 1957 and is called Lawson criterion. This task is so complex that most nuclear fusion reactors produce less energy than they consume, making them highly impractical.

A charged particle moving perpendicular to a magnetic field executes a circular path around the magnetic field. If the particle also has some motion (speed) along the field line, the circular path turns into a helical path. The strength of the magnetic field and mass, charge, and speed of the charged particle dictate the radius of the helical path. Thus, the charged particles get trapped in a magnetic bottle. A doughnut-shaped magnetic chamber, known as a tokamak (abbreviated from the Russian toroid-kamera-magnit-karushka, or toroidal chamber with magnetic coil), is commonly used to confine plasma. Two noted Russian scientists, Igor Tamm and Andrei Sakharov, contributed much to this controlled thermonuclear reactor design.

As the charged particles move, they establish current through the plasma and the charged particles collide with each other, particularly those with opposite charges that move in opposite directions. These collisions create heat, which in turn increases the resistance and decreases the current. The system reaches an equilibrium state in time. To increase the temperature further, external heating devices are used. Neutral beam injection and high-frequency electromagnetic waves are some of the techniques used to increase temperature. Laser-induced fusion, whereby hydrogen droplets are bombarded by high-energy laser beams from all sides, has also been used. This compression increases the temperature of the droplet. This is a much simpler process than the tokamak process. In May 2009, the Lawrence Livermore National Laboratory announced the development of a high-energy laser system that can heat hydrogen atoms to temperatures that exist in the cores of stars.

The Tokamak Fusion Test Reactor (TFTR) at Princeton, New Jersey, the Joint European Torus (JET) at Culham, England, and the JT-60 (JT for Japan Torus) in Naka, Japan, emerged as three leading facilities for fusion research. In 2010, the Japanese facility was disassembled to be upgraded to JT-60SA by using superconducting coils. In 2013, a consortium of scientists from the European Union, India, China, Japan, Korea, Russia, and the United States began construction on the 500-megawatt International Thermonuclear Experimental Reactor (ITER) in France. This reactor was designed to fuse deuterium and tritium and required a hot plasma of about 1.5 × 10⁸ kelvin in a confined space. While earlier tokamaks could only achieve fusion for a fraction of a second, this new design aimed to allow scientists to sustain the fusion reaction for extended periods. Another important development was the Wendelstein 7-X, which became the world's largest stellarator fusion device upon completion in 2014; its first plasma was produced in 2015. In 2016, it was reported that China's Experimental Advanced Superconducting Tokamak (EAST) had managed to sustain plasma at a temperature hotter than the core of the sun for 102 seconds.

In 2022, scientists at the Lawrence Livermore National Laboratory made a historic breakthrough in nuclear fusion research by creating a nuclear fusion reaction that resulted in a net gain of energy. While researchers agreed that it would still be many more years before the discovery could be put into practical use, the fact that scientists were able to use nuclear fusion to create more energy than what was started with showed promise for large-scale energy production in the future.

Environmental Advantages and Concerns

The natural product resulting from the fusion of deuterium with tritium is helium, which poses no threat to life and does not contribute to global warming. Although tritium is radioactive, it has a short half-life of twelve years, has a very low amount of decay energy, and does not accumulate in the body. It is cycled out of the human body as water, with a biological half-life of seven to fourteen days. Since fusion requires precisely controlled conditions of temperature, pressure, and magnetic field parameters in order to generate net energy, there is no danger of any catastrophic radioactive accident, as heat generation in a fusion reactor would quickly stop if any of these parameters were disrupted by a reactor malfunction. In addition, since the total amount of fusion fuel in the reactor vessel is very small (a few grams) and the of the plasma is low, there is no of a runaway reaction, because fusion would cease in a few seconds if fuel delivery were stopped.

Because of the generation of high-energy neutrons in the deuterium-tritium reaction, the typical structural materials of fusion reactors, such as stainless steel or titanium, tantalum, and niobium alloys, will become radioactive when bombarded by the neutrons. The half-lives of the resulting are typically less than those generated by nuclear fission. Most of this radioactive material would reside in the fusion reactor core and would be dangerous for about fifty years. Low-level wastes would be dangerous for about one hundred years. Since the choice of materials used in a fusion reactor is quite flexible, low-activation materials, such as vanadium alloys and carbon fiber materials, that do not easily become radioactive can be used.

Most fusion reactor designs use liquid lithium as a coolant and for generating tritium when it is bombarded by neutrons coming from the fusion reaction. Because lithium is highly flammable, a fire could release its tritium contents into the atmosphere, posing a risk. Estimates of the amount of tritium and other radioactive gases that might escape from a typical fusion power plant indicate that they would be diluted to safe levels by the time they reached the perimeter fence of the plant.

Another safety and environmental concern associated with fusion reactors is the potential that the neutrons generated in a fusion reactor could be used to breed plutonium for an atomic bomb. In order for a reactor to be used in this way, however, it would have to be extensively redesigned; thus the plutonium production would be very difficult to conceal. The tritium produced in fusion reactors could be used as a component of hydrogen bombs, but the likelihood is minimal.

Conclusion

The technology to produce uncontrolled nuclear fusion was mastered in the twentieth century. However, the technology to control fusion reactions for extended periods still eludes scientists. The recent significant improvements in maintaining plasma temperature and confinement have nevertheless raised the prospect that one day nuclear fusion might be used for electricity production. Some scientists also continue to work toward the possibility of cold fusion, a theoretical concept that would provide energy without the need for high temperatures and without the potentially harmful radiation produced by standard nuclear reactions.

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